Relating Accretion Rate
and Jet Power in Elliptical
Galaxies
Robert Dunn
Institute of Astronomy, Cambridge
Steve Allen (KIPAC), Andy Fabian (IoA), Greg Taylor
(UNM) & Chris Reynolds (Maryland)
MNRAS, in press, astro-ph/0602549
AGNs in Ellipticals
• The observed luminosities of AGN in ellipticals are also much
lower than those expected from standard Bondi accretion
(assuming an efficiency of 10%).
• Di Matteo et al. 2003 and Taylor et al. 2005 investigated this in
M87, NGC4696 & NGC6166.
– Although the bolometric luminosities are a factor ~1000 below
the Bondi rates, the powers inferred from X-ray cavities are
comparable (to a factor ~few).
Taylor et al. 2006, 5GHz & 8GHz radio contours on Chandra X-rays.
Taylor et al. 2006, 5GHz & 8GHz radio contours on Chandra X-rays.
AGNs in Ellipticals
• The observed luminosities of AGN in ellipticals are also much
lower than those expected from standard Bondi accretion
(assuming an efficiency of 10%).
• Di Matteo et al. 2003 and Taylor et al. 2005 investigated this in
M87, NGC4696 & NGC6166.
– Although the bolometric luminosities are a factor ~1000 below
the Bondi rates, the powers inferred from X-ray cavities are
comparable (to a factor ~few).
• The origin of the galaxy luminosity function and the mechanism
which truncates the star formation in the largest ellipticals is still a
subject for debate (e.g. Benson et al. 2003, Croton et al. 2005).
Accretion Rates
• The exquisite resolution of Chandra allows regions close to the
Bondi radius to be resolved.
• The Bondi accretion rate is
•
M Bondi
π cs ρ r
= π (GM BH )2c ρ =
4
−3
s
2
A
• We chose a sample of nine ellipticals which were all closer than
dL~100Mpc.
• As a result for an efficiency η, the Bondi accretion power is
•
PBondi = η M Bondi c 2
X-ray Bubbles
• When AGN are embedded in a
thermal plasma, e.g. in
clusters, the relativistic jets
inflate cavities of radioemitting plasma.
• These cavities are buoyant
and detach from the central
AGN and rise up through the
thermal plasma.
• These have been termed
“bubbles.”
Fabian et al. 2006, Perseus Cluster, 330MHz radio contours
X-ray Cavity measurements
• The energy contained within these bubbles can be estimated from
the work done in inflating the cavities; E=4pV
– We ignored the extra energy which goes into sound waves
(e.g. Churazov et al. 2002) as it is likely to be small.
• As most of the bubbles in these ellipticals are still attached to
their radio cores, they must be young and so we estimate their
ages assuming that they are expanding at the sound speed.
• Hence
4 pV 4 pV
Pjet =
=
t cs
R cs
Results
Allen, Dunn et al. 2006, MNRAS, in press, astro-ph/0602549
• Fitting the clear correlation with a power-law
log( PBondi / 10 43 erg/s) = A + B log(Pjet / 1043 erg/s)
•
A=0.65±0.16, B=0.77±0.20, using an estimator than accounts for
errors on both axes (BCES(Y|X) from Akritas & Bershady 1996).
• The plotted quantities have some variables in common – dL T and ρ
.
– However, none of these could introduce the observed positive
correlation.
Implications
• The correlation means that a significant fraction (~2%) of the
energy associated with the rest mass of the material entering the
Bondi accretion radius emerges in relativistic jets.
• This tight correlation suggests that the Bondi formalism provides a
good description of the accretion process.
• Most of the material which passes through the accretion radius
flows all the way down to the black hole.
– Little material is lost from the accretion flow en route to the
region of jet formation
• The accretion flows must be stable over the lifetimes of the bubble
~107 years.
• The observed efficiency of the feedback of the accreted matter is
similar to that assumed in semi-analytic models (e.g. Croton et al.
2005).
• For the galaxies observed here, the power fed back into the X-ray
gas is sufficient to offset the cooling, and so prevent further star
formation.
• In clusters, however, the cooling rate is much larger (~1045 erg/s)
– If the black hole masses and accretion rates are not much
larger then those in these ellipticals, then it is unlikely that the
jet power will be sufficient to balance cooling.
Summary
• There exists a tight correlation between the Bondi accretion rates
and the Jet power
43
43
log(
P
/
10
erg/s
)
=
A
+
B
log(
P
/
10
erg/s)
log(Bondi
P
/ 10 43 erg/s) = A + B log( Pjet / 10 43 erg/s)
Bondi
•
jet
A=0.65±0.16, B=0.77±0.20
• A significant fraction (~2%) of the energy associated with the rest
mass of the material entering the Bondi radius emerges in
relativistic jets.
• The Bondi formulae provide a good description of accretion
• In ellipticals, the black holes feed back sufficient energy to stem
cooling and star formation.